Wherever there is a source of nutrition and moisture, microorganisms will grow. This is often the case even in extreme environmental conditions. These microorganisms may spoil food, interfere with manufacturing processes, decay consumer products, or affect human health. Consumer products require the substantial elimination of these unwanted microorganisms, since these products are frequently the delivery means for bringing the microorganisms into or onto the human body. Consumer products that can carry microorganisms include paper products, foodstuffs, clothing, medical devices, cosmetics, and a multitude of other consumer health and hygiene products.
In addition to the need for reducing the number of microorganisms on consumer products, recent bioterror attacks have prompted the need for the decontamination of products that have been purposefully tainted with microorganisms. Aerosolized anthrax spores (Bacillus anthracis) have been inserted in and upon articles of mail, resulting in infection and death. Thus, there is a need to protect postal employees and potential recipients of microorganism-tainted letters and packages sent through the mail.
Control of microbial growth can be achieved by inhibition of growth, killing, or removing the microorganisms from the environment. Pasteurization and sterilization are common means to control microbial growth. Pasteurization is the process of removing or reducing the level of harmful bacteria from a product. Sterilization is the process of killing or removing all living organisms and viruses from a product. Microorganism death is an exponential function; therefore it is linear when plotted on a log scale. The rate of microbial destruction is termed an “x-log reduction” or “x-log kill,” which corresponds to the percentage of microorganisms killed or inactivated. For example, if 99% of all microorganisms in or on a given product have been killed or inactivated, this is equivalent to a 2-log kill. If 99.9% of all microorganisms in or on a given product have been killed or inactivated, this is equivalent to a 3-log kill; and so on. A 6-log kill, wherein 99.9999% of all microorganisms have been killed or inactivated, is the destruction rate at which a product is considered “sterilized.”
Several methods exist to assist in the decontamination of microorganisms from products. For example, many manufacturing processes employ biocides in process streams and/or post-process treatment of the finished product. However, process stream biocide treatments can be ineffective and harmful to the environment. Other methods include autoclave sterilization, which involves using steam under pressure. The high heat and pressure of autoclave sterilization destroys many pathogenic bacteria. However, steam heat cannot be used in many applications because the elevated temperatures and wet environment would destroy the aesthetic and functional properties of the product.
Electromagnetic irradiation is another method for removing or reducing the amount of microorganisms in or on products. Microwaves, ultraviolet (UV) radiation, X-rays, and gamma rays from sources such as Cobalt 60 are commonly used to reduce the number of pathogens in or on products. UV irradiation, which does not penetrate solid, opaque, or light absorbing materials, can be useful for decontaminating ambient air, surfaces, and liquids that do not absorb the UV waves. Gamma rays and X-rays are more penetrating, and can be used to decontaminate a wider range of products. However, these decontamination methods are relatively more expensive, difficult to use, and require a substantial amount of shielding between the workers and the radiation source. Another significant disadvantage of gamma irradiation is the relatively long exposure time required to effectively decontaminate the product. This, coupled with the high cost, makes irradiation an impractical method in many high-throughput manufacturing and process streams where rapid decontamination is desired.
Yet another method for inactivation or elimination of microorganisms in or on products is through the use of sonication. Sonication is the use of high power, low frequency ultrasound with a liquid medium as the ultrasonic wave carrier. The liquid medium acts as a physical coupling device that transmits the ultrasonic energy through the medium and onto the product to be decontaminated. Similar to autoclave sterilization, however, immersing the product into a liquid medium may destroy its aesthetic and functional properties.
The use of ultrasonic energy for microbial decontamination without the use of a physical coupling device is also known. For example, in U.S. Pat. No. 6,576,188, Rose et al. describe the simultaneous exposure of an item to ultraviolet light and ultrasonic wave energy in a non-aqueous environment, such as air, to eliminate microbial contamination. By way of further example, U.S. Patent Application 2004/0028552 describes the use of ultrasonic energy for microbial decontamination using a “non-contact” ultrasonic apparatus in ambient air. However, due to the significant attenuation of the ultrasound energy when transmitted through a compressible medium, such as gas or ambient air, in addition to being an ineffective method of applying ultrasound, the process is sensitive to the physical distance between the acoustic source and the treated materials as well as the materials' orientation to the acoustic waves.
These examples, however, do not achieve a significant rate of killing at a rapid enough pace for many manufacturing and process streams. Moreover, destroying microorganisms on products using these processes may harm the aesthetic and functional properties of the product.
Therefore, it would be desirable to provide a process for the destruction of microorganisms on a product in a relatively short period of time and with a relatively high microorganism destruction rate. It would also be desirable to provide a process for the destruction of microorganisms on a product in a relatively short period of time and with a relatively high microorganism destruction rate, without damaging the aesthetic and functional properties of the product.